The present invention relates to a process for coating a substrate. More particularly, the invention relates to coating a substrate with a tin oxide-containing material, preferably an electrically conductive tin oxide-containing material.
The present invention, in addition, relates to field dependent fluids, more particularly to improved electric field dependent fluids, wherein the fluids exhibit a reversible rise in viscosity and a corresponding change in sheer stress in the presence of an electric field.
An application where substrates with coatings, e.g., electrically conductive and/or polarizable coatings can find particular usefulness are field dependent fluids which exhibit a reversible viscosity rise in the presence of an applied electric field which viscosity can be pronounced particularly by exhibiting resistance to sheer in response to the application of an electric field. The electric field dependent fluids are generally known as electro viscous and/or electro rheological fluids and have been referred to in the prior art as exhibiting the Winslow Effect. The fluids generally comprise a non-conductive liquid having a low to a high dielectric constant and having a suspension of solid particles, particularly finely divided particles.
In many of the prior art fluids, water has been incorporated into the fluid as a critical element to achieve the desired viscosity change. Such fluids are described in U.S. Patents to Winslow for example U.S. Pat. Nos. 2,147,850, 2,661,596, 2,661,825 and 3,047,507. In U.S. Pat. No. 2,661,596 a micronized silica gel powder is combined with an electrically stable oily vehicle and about 15 parts by weight of water. In addition, various oil soluble dispersing agents and water soluble dispersing agents are incorporated into the electro viscous fluid. In addition, certain gelatinous metallic hydrates of ferric oxide and stannous oxide can be substituted for certain of the water soluble ingredients.
A number of alternatives to silica have been investigated such as alumina, silica alumina in combination with a surface active agent, an amine, fatty acid and water, as disclosed in U.S. Pat. No. 3,367,872 to Martinek, et al, and the use of particulate conductive metal such as copper iron, aluminum, zinc, bronze and lead as disclosed in U.S. Pat. No. 3,385,793 to Klass, et al. A variety of polar materials including water were used by Klass, et al, to activate the electro viscous fluids. In addition, various amines and lower hydroxy hydrocarbons were found to be efficient as activators.
The use of water activated systems in general limits the effective upper temperature of the system to about 70.degree. C. At temperatures in excess of 70.degree. C. fluids gradually lose water and thus, a reduced effect occurs. The problems associated with water activated systems and/or low boiling alcohol, amine and hydroxy type activated systems have been approached through the use of certain solid particulates which are substantially free of water. For example, U.S. Pat. No. 4,744,914 discloses a dispersed particulate crystalline zeolite material, whereas U.S. Pat. No. 4,879,056 discloses a dispersed particulate organic polymeric polyelectrolyte material. Such zeolite and organic polyelectrolyte materials are substantially free of adsorbed water. In addition to these materials, Block, et al, U.S. Pat. No. 4,687,589, prefers organic semi-conductors which comprise an unsaturated, fused polycyclic system as a particulate solid in an insulating fluid. In the review article Electrorheology by Block, et al, Journal of Physics, D, Applied Physics, Vol. 21, 1988, at pages 1661-1677, Block, et al, reviews the various compositions, their deficiencies and their preference for certain organic semi-conductors.
A number of applications have been proposed for field dependent fluids, i.e., eletrorheological fluids including shock absorbers, variable speed transmissions, clutch mechanisms, torque converters and various other applications in dampening, robotics and hydraulics.
In many of the above-noted applications it would be advantageous to have an electrically, electronically conductive and/or electro mechanical particulate which is substantially uniform, has system designed electronic conductivity and/or polarizability has good chemical properties, e.g., morphology, stability, has good mechanical properties and has good fluid compatability and stability.
In addition, in many of the above noted applications it would be advantageous to have particulates which are electrorheologically effective in fluids over a wide temperature range, including temperatures above 70.degree. C. and which can exhibit improved particle integrity during use while being reversibly cycled in viscosity.
A number of techniques may be employed to provide conductive tin oxide coatings on substrates. For example, the chemical vapor deposition (CVD) process may be employed. This process comprises contacting a substrate with a vaporous composition comprising a tin component and a dopant-containing material and contacting the contacted substrate with an oxygen-containing vaporous medium at conditions effective to form the doped tin oxide coating on the substrate. Conventionally, the CVD process occurs simultaneously at high temperatures at very short contact times so that tin oxide is initially deposited on the substrate. However tin oxide can form off the substrate resulting in a low reagent capture rate. The CVD process is well known in the art for coating a single flat surface which is maintained in a fixed position during the above-noted contacting steps. The conventional CVD process is an example of a "line-of-sight" process or a "two dimensional" process in which the tin oxide is formed only on that portion of the substrate directly in the path of the tin source as tin oxide is formed on the substrate. Portions of the substrate, particularly internal surfaces, which are shielded from the tin oxide being formed, e.g., such as pores which extend inwardly from the external surface and substrate layers which are internal at least partially shielded from the depositing tin oxide source by one or more other layers or surfaces closer to the external substrate surface being coated, do not get uniformly coated, if at all, in a "line-of-sight" process. Such shielded substrate portions either are not being contacted by the tin source during line-of-sight processing or are being contacted, if at all, not uniformly by the tin source during line-of-sight processing. A particular problem with "line-of-sight" processes is the need to maintain a fixed distance between the tin source and the substrate. Otherwise, tin dioxide can be deposited or formed off the substrate and lost, with a corresponding loss in process and reagent efficiency.
One of the preferred substrates for use in field dependent fluids are inorganic substrates, in particular particles such as flakes, spheres, fibers and other type particles. Although the CVD process is useful for coating a single flat surface, for the reasons noted above this process tends to produce non-uniform and/or discontinuous coatings on non-flat, non-equidistant surfaces and/or three dimensional surfaces having inner shielded surfaces and/or the processing is multi-step and/or complex and/or time consuming. Such non uniformities and/or discontinuities and/or processing deficiencies are detrimental to the electrical and chemical properties of the coated substrate. A new process, e.g., a "non-line-of-sight" or "three dimensional" process, useful for coating such substrates would be advantageous. As used herein, a "non-line-of-sight" or "three dimensional" process is a process which coats surfaces of a substrate with tin oxide which surfaces would not be directly exposed to tin oxide-forming compounds being deposited on the external surface of the substrate during the first contacting step and/or to improve the processability to conductive components and articles and/or for the type of substrate to be coated. In other words, a "three dimensional" process coats coatable substrate surfaces which are at least partially shielded by other portions of the substrate which are closer to the external surface of the substrate and/or which are further from the tin oxide forming source during processing, e.g., the internal and/or opposite side surfaces of glass or ceramic particles such as fiber or spheres, or flakes or other shapes or surfaces.
Although a substantial amount of work has been done, there continues to be a need for a new method for coating substrates, particularly three dimensional substrates with tin oxides. The prior art processes described below follow conventional processing techniques such as by sintering of a tin oxide and/or the instantaneous conversion to tin oxide by spray pyrolysis.
For example in "Preparation of Thick Crystalline Films of Tin Oxide and Porous Glass Partially Filled with Tin Oxide," R. G. Bartholomew et al, J. Electrochem, Soc. Vol. 116, No. 9, p1205(1969), a method is described for producing films of SnO.sub.2 on a 96% silica glass substrate by oxidation of stannous chloride. The plates of glass are pretreated to remove moisture, and the entire coating process appears to have been done under anhydrous conditions. Specific electrical resistivity values for SnO.sub.2 -porous glass were surprisingly high. In addition, doping with SbCl.sub.3 was attempted, but substantially no improvement, i.e., reduction, in electrical resistivity was observed. Apparently, no effective amount of antimony was incorporated. No other dopant materials were disclosed.
In "Physical Properties of Tin Oxide Films Deposited by Oxidation of SnCl.sub.2," by N. Srinivasa Murty et al, Thin Solid Films, 92(1982) 347-354, a method for depositing SnO.sub.2 films was disclosed which involved contacting a substrate with a combined vapor of SnCl.sub.2 and oxygen. Although no dopants were used, dopant elements such as antimony and fluorine were postulated as being useful to reduce the electrical resistivity of the SnO.sub.2 films.
This last described method is somewhat similar to the conventional spray pyrolysis technique for coating substrates. In the spray pyrolysis approach tin chloride dissolved in water at low pH is sprayed onto a hot, i.e., on the order of about 600.degree. C., surface in the presence of an oxidizing vapor, e.g., air. The tin chloride is immediately converted, e.g., by hydrolysis and/or oxidation, to SnO.sub.2, which forms a film on the surface. In order to get a sufficient SnO.sub.2 coating on a glass fiber substrate to allow the coated substrate to be useful as a component of a lead-acid battery, on the order of about 20 spraying passes on each side have been required. In other words, it is frequently difficult, if not impossible, with spray pyrolysis to achieve the requisite thickness and uniformity of the tin oxide coating on substrates, in particular three dimensional substrates.
Dislich, et al U.S. Pat. No. 4,229,491 discloses a process for producing cadmium stannate layers on a glass substrate. The process involved dipping the substrate into an alcoholic solutio of a reaction product containing cadmium and tin; withdrawing the substrate form the solutio in a humid atmosphere; and gradually heating the coated substrate to 650.degree. C. whereby hydrolysis and pyrolysis remove residues from the coated substrate. Dislich, et al is not concerned with coating substrates for lead-acid batteries, let alone the stability required, and is not concerned with maintaining a suitable concentration of a volatile dopant, such as fluoride, in the coating composition during production of the coated substrate.
Pytlewski U.S. Pat. No. 4,229,491 discloses changing the surface characteristics of a substrate surface, e.g., glass pane, by coating the surface with a tin-containing polymer. These polymers, which may contain a second metal such as iron, cobalt, nickel, bismuth, lead, titanium, canadium, chromium, copper, molybdenum, antimony and tungsten, are prepared in the form of a colloidal dispersion of the polymer in water. Pytlewski discloses that such polymers, when coated on glass surfaces, retard soiling. Pytlewski is not concerned with the electrical properties of the polymers or of the coated substrate surfaces.
Gonzalez-Oliver, C. J. R. and Kato, I. in "Sn (Sb)-Oxide Sol-Gel Coatings of Glass," Journal of Non-Crystalline Solids 82(1986) 400-410 North Holland, Amsterdam, describe a process for applying an electrically conductive coating to glass substrates with solutions containing tin and antimony. This coating is applied by repeatedly dipping the substrate into the solution of repeatedly spraying the solutio onto the substrate. After each dipping or spraying, the coated substrate is subjected to elevated temperatures on the order to 550.degree. C.-600.degree. C. to fully condense the most recently applied layer. Other workers, e.g., R. Pryane and I. Kato, have disclosed coaTing glass substrates, such as electrodes, with doped tin oxide materials. The glass substrate is dipped into solution containing organo-metallic compounds of tin and antimony. although multiple dippings are disclosed, after each dipping the coated substrate is treated at temperatures between 500.degree. C. and 630.degree. C. to finish off the polycondensation reactions, particularly to remove deleterious carbon, as well as to increase the hardness and density of the coating.